Electrical motor control system



Oct. 7, 1947.

J. E. REILLY ELECTRICAL MOTOR CONTROL SYSTEM Filed June 16, 1945 ZZZ 76 8e WITNESSES: INVENTOR ]acK./ e///y.

.41/ 9" WWI/0S2 Patented Oct. 7, 1947 ELECTRICAL MOTOR (IQNTROL SYSTEM Jack E. Reilly, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Appl cation June 16 5.. Serial N 599,837

5 Claims.

This invention relates to electrical systems and in particular to are furnace regulatin systems. An object of this invention is to provide in an electronic are furnace regulating system, for automatically limiting the current flow to the electrode'motor [to protect the electric valves 1 utilized in controlling the operation of the motor.

Another ohi'ect of this invention is to provide an electronic control system for a reversible motor whereby the flow of current to the motor is limited to protect the electric valves utilized in the system.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing the single figure of which is a diagrammatic view ofthe apparatus and circuits. of an arc furnace regulating system embo yin the teachings of this invention.

Referring to. the drawing thisinvention is illustrated by reference to an arc furnace III havina electrodes I2, I4 and I6 disposed therein to he positioned with respect to a metal char e 18. The electrodes L2, I4 and [6 are connected by conductors 2 B and 24, respectively, to a source of supply inot shown). Since the control system and apparatus utilized in positioning each oi the electrodes is the sarne, only the system and appara tus associated with electrode Lo is illustrated,

it being understood that similar control systems I are tobe provided for other electrodes.

As illustrated, a reversible motor 26 is utilized for positioning the electrode I2 the electrode I2 being connected to be. raised or lowered by operation of the motor. For this purpose a winding drum 28 is disposed to be driven by the motor I10, a, flexible cable 30 being connected to the electrode L2 and disposedto be wound uponflthe drum 28. The motor comprises an armature winding 32- anrl a fieldwindins 3,4, the field 1 winding being disposed to be separately excited from a source oi supply (not shown).

In order to control the direction and speed oi operation ofthe motor 26, two sets of electric valv s, such'as: th thyratron tubes li -136 an 4 ar uti z d- Thevalves 5. 38, 44 and!!! areiprovided with anodes 44,46, lliam 5.0 r spectively, connected bathe terminals .of the secondary windings 0t transiormrs '52 and 54 as connected across supply'conductors 56 and 5,8. The valves 36, 38, 40 and 42 are also provided with cathodesdll. 62,164and 65, respectively, the cathode 6.0 ndlfi b ms connected throughcon- I4, through resistor I6, conductors I8 and 80, armature winding 32, conductors 82 and 84, resistor 86 and conductor 68 to the center tap 90 of the secondary winding of transformer 52. In a similar manner the cathodes 64 and 66 are qnnected by conductors 02 and 94-96, respectively, to conductor 98, through resistor I00, conductors 84 and 82, armature winding 32, conductors 80 and [8, resistor I02 and conductor I04 to the center tap [D6 of the secondary winding of transformer 54.

The valves 36, 38, 4,0 and 42 are also provided with grids [08, H0, H2 and H4, respectively. The grids I08 and III) are supplied with an alternating-current potential from the supply condoctors 56 and 58 through a transformer IIB, a phase-shifting circuit I I8 and a grid transformer I20. The secondary winding of the grid transformer comprises two sections I22 and I24, the winding 1 22 having one end connected through a resistor I26 to the grid H0 and the other end connected through a conductor I28, a part of resistor I30 and the conductor 12 to the cathode 62 of valve 38 whereas the winding I24 has one end connected through resistor I32 to the grid I00 and the other end connected through conductor I34, resistor I36 and conductors I4, 10

and 68 to the cathode 60 of the valve 36.

Likewise, an alternating-current potential is supplied to. the grids H2 and I M of valves 40 and 42, respectively, from supply conductors 56 and 66 through transformer I40, the phase-shifting circuit I42 andtthe grid transformer I44 whlchis provided with two secondary winding I46 and I48. The winding I461 has one end connected through resistor I50 to the grid II 4 of valve 42, the other end of winding I46 being connected through conductor 1-52, resistor I53 and condoctors 98, 96 and 94 to the cathode 66.

The secondary winding I48 is also connected at one endthroughresistor I54 to the grid II2 of valve 40, the other endpbeing connected through conductcr iifitafpart oi resistor I58 and conductor to. the cathode, 64.

trolling the grid-cathode potential of the valves illustrated, the primary windings of which are fi the resistor I60 is connected through conducin a predetermined manner. Thus one end of torstiii and, IO-J2 to cathodes 60 and 62, respectively, and aflexible lead I84 connected to an intermediate tap of resistor I60 is connected ductorsfia lin andllrespectively, to conductor Q throu h r i tor "5 on ctor 68, resistor I10 aaases and conductor I34 to the secondary winding I24, the conductor I68 also being connected through a, part of resistor I30 and conductor I28 to the secondary winding I22 of the grid transformer I20. In the same manner one end of resistor I62 is connected through conductors 94 and 86-92 to the cathodes 66 and 64, respectively, of valves 42 and 40, respectively, and a flexible lead I12 connected to an intermediate tap of resistor I62 is connected through resistor I'I4, conductor I16, resistor I18 and conductor I52 to the secondary winding I46, the conductor II6 also being connected through a part of resistor I58 and conductor I56 to the secondary winding I48 of transformer I44.

In order to control the direct-current biasing potential for the discharge valves 36-30 and 40-42, vacuum type discharge valves I80 and I82, respectively, are provided being disposed to control the current flow through the potentiometer resistors I60 and I62, respectively. The valves I80 and I82 comprise anodes I84 and I85, respectively, grids I86 and I08, respectively, and cathodes I90 and I92, respectively. The anodes I84 and I85 are connected by conductors I06 and I08, respectively, to an output terminal of associated rectifiers 200 and 202, respectively, which are supplied from transformers 204 and 208, respectviely, the primary windings of the transformers being connected across supply conductors 56 and 51.

The cathode I90 of discharge valve I80 is connected through a self-biasing resistor 200, a flexible lead 2I0 connected to an intermediate tap of a resistor 2I2, resistor I02, conductor I0, resistor I6, conductors I4 and I0, potentiometer resistor I60, conductor 2M and a smoothing reactor 2I6 to the other output terminal of the rectifier 200. In a similar manner, the cathode I92 of valve I82 is connected through a selfbiasing resistor 2 I8, a flexible lead 220 connected to an intermediate tap of a resistor 222, resistor 06, conductor 84, resistor I00, conductors 98 and 96, potentiometer resistor I62, conductor 224 and a smoothing reactor 226 to the other output terminal of the rectifier 202.

In order to control the biasing potential of the grids I06 and I80 of the discharge valves I and I82, respectively, the grids I86 and I88 are connected so that their potential is controlled in response to the current flowing through the electrode I2 and the are potential across the electrode arc. Thus the grid biasing circuit of the valve I80 extends from the grid I86 through a grid resistor 228, conductor 230, series connected resistors 232 and 234, conductor 236, resistor I6, conductor I8, resistor I02, a part of resistor 2I2, flexible lead 2I0 and self-biasing resistor 208 to the cathode I90.

The resistors 232 and 234 are control resistors and are disposed to have a direct-current voltage thereacross proportional to the are potential and the flow of current through electrode I2, respectively. For this reason the resistor 232 is connected across a rectifier bridge 238 which is connected through a transformer 240, the terminals of the primary winding of which are connected by conductors 242 and 244 to supply conductor 20 and the receptacle of the grounded furnace I0, respectively. The control resistor 234 is also connected across a rectifier bridge 246 which is supplied by a transformer 248 connected to be energized by the current transformer 250 in accordance with th current flowing in conductor 20. The transformers 240 and 248 are preferably 4 of a one-to-one ratio and are employed primarily to prevent sneak circuits or eifects on the grid of the discharge valves I and I82.

As illustrated the rectifier bridges 238 and 246 are connected in opposition whereby the directcurrent voltages across resistors 232 and 234 are of opposite polarity. Thus for any variation in the flow of current through the electrode and in the are potential, the differential of the directcurrent voltages across the series connected resistors 232 and 234 normally controls the directcurrent biasing potential on the grid I06 of the discharge valve I00.

The grid I88 of the discharge valve I02 is likewise connected through a grid resistor 252, conductor 254, series connected resistors 256 and 250, conductor 260, resistor I00, conductor 84, resistor 86, a part of resistor 222, flexible lead 220 and the self-biasing resistor 2I0 to the cathode I92. The control resistor 256 is connected across a rectifier bridge 262 the input terminals of which are connected by conductors 242 and 244 to the supply conductor 20 and the receptacle of the grounded furnace I0, respectively, whereby a direct-current voltage proportional to the arc potential is impressed across resistor 256. The control resistor 258 is also connected across a rectifier bridge 264, the input terminals of which are connected across the current transformer 250 on the conductor 20 whereby the direct-current voltage impressed across resistor 250 is proportional to the current flowing through the electrode I2. The rectifier bridges 262 and 264 are so connected across the resistors 256 and 258, respectively, that the direct-current voltages impressed thereacross are of opposite polarity for normally controlling the direct-current biasing potential of the grid I88 of the valve I82.

In the system described, the resistors 208 and 2I'8 are selected so as to assure a negative bias on the grids of the valves I80 and I02, respectively, when the electrode I2 is in an ideal or balanced operating position and the direct-current voltages across the control resistors are so balanced that the differentials of the direct-current voltages are substantially zero. The resistors I60 and I62 in the circuits with the sets of discharge valves 36-38 and 40-42, respectively, are so adjusted by adjusting the flexible leads I64 and I12, respectively, that the drop across the section of the resistors I60 and I62 in circuit with the grids of the valves 36-38 and 4042, respectively, gives a sufiicient dead zone to prevent simultaneous firing of both sets of the discharge valves.

The resistors 16 and I00 in the motor armature circuits controlled by the firing of valves 36-38 and 40-42, respectively, are also connected in the grid circuits of the valve I00 and I82, respectively, and function to automatically limit the starting current of the motor to a safe value for the valves 36-30 and 40-42. In addition to limiting the starting current of the motor, the resistors I6 and I00 also function to protect the valves 36-38 and 4042, respectively, from any overload on the system by so controlling the grid bias of the valves that the current flow to the motor is within the safe current limits of the valves. The specific manner in which such protection is obtained will be fully explained hereinafter in connection with the operation of the system.

In operation, assuming that the system is energized, the electrodes I2, I4 and I6 are positioned with respect to the metal charge I8 in the misses furnace by a manual. or automatic manipulation (not shown) of the? electrodes, such positioning and methods of doing so being wen known in the art. Assuming that the initial positioning of the electrodes is to be obtained automatically, as soon as the system is energized and conductors 20, 22 and 24' are connected to a source of power supply, a potential eriists from the electrode I2 to the metal I8 in the furnace. This potential is at a maximum and since the electrodes I2, I4 and I6 are not as yet adjusted, there is no current flow. Thehigh value of the are potential impresses a high value of direct-current voltage across each of the control resistors 232 and 256, and since current is not flowing through the electrode I2, the direct-current voltages across resistors 234 and 258 proportional to the flow of current are zero. The direct-current voltage across control resistor 232 places a large negative bias on the grid I86 of valve I80 whereas the direct-current voltage across control resistor 256 places a less negative or more positive bias on the grid I88 of valve I82.

The bias thus impressed on the grid I88 is such that the valve I82 is rendered more conductive with the result that current flows from the negative terminal of rectifier bridge 202,

iii

through reactor 226, conductor 224, potentiometer resistor I62, conductors 36 and 98, resistor I00, conductor 84, resistors 86 and 222, flexible lead 220, self-biasing resistor 2I8, cathode I92 and anode 185 of valve I82 and conductor I88 to the positive terminal of the supply rectifier bridge 202. Since the direct-current voltage proportional to the are potential is at a maximum, the conductivity of the valve I82 is also a maximum and maximum current flows through the potentiometer resistor I62. The flow of current through resistor I62 renders the direct-current biasing potential on the grids H2 and N4 of valves 40 and 42, respectively, more negative whereby the grid-cathode potential of the valves is lowered and maintained at a potential below the critical potential of the valves and the valves 40 and 42 are blocked with the result that no current flows therethrough to the motor 26.

At the same time, the large direct-current voltage proportional to the are potential impressed across control resistor 2.32 cooperates with the self-biasing resistor208 to Impress a more negative biasing potential on the grid I86 to decrease the conductivity of valve I80. Thus the current flow through potentiometer resistor I60 in the anode-cathode circuit of valve I80 is so decreased that the direct-current biasing potential for the valves 36 and 38 is so reduced that the resultant grid-cathode potential of valves 36 and 38 rises above the critical potential of the valves and they become highly conductive to pass current to the motor 26. This conducting circuit extends from the center tap 90 of the secondary winding of transformer 52 through conductor 88, resistor 88, conductors 84 and 82, armature windings 32- of the motor 28, conductors 80 and 1-6, resistor I6, conductor 14, the parallel conductors l-68 and I2 to cathodes 60 and 62, respectively, of valves 36 and 38, anodes 44 and 46, respectively, and from thence to the terminals of the secondary winding of transformer 52, to so energize the motor 26 as to effect the operation thereof in a direction to lower the electrode I2. Since theother electrodes I4 and iii are as yet not in contact with the metal bath I8, the electrode I2 is lowered to a position where it ongages the metal I8,

As the electrode 12' is owered the potential be} tvveen the electrode I2 and the metal I8 decreases with the result that the direct-current voltage across resistors 232 and 258 is" so decreased that the'bias' on the grids of valves I88 and n2 is rendered less negative and more negative, respectively. By rendering the biasing potential of the grid of valve I82 more negative the current flowing through the resistor I62 for controlling the biasing potential of the valves 48 and 42 more nearly approaches the value where the discharge devices 40 and 42 can be rendered conductive.

As soon as the electrodes I2, I4 and [6 are actuated to a position where a circuit is complete throughthe electrodes in conjunction with the metal I8, maximum current flows through the electrode I2 and a direct-current voltage proper: tlonal to the current is imposed across each of the control resistors 234 and 258 with the resultthait, the differential of the direct-current voltages; across resistors 232-234 and 256-258 is sucli; as to place a less negative bias on the grid of valve I and a more negative bias on the grid of valve I82.

The less negative or more positive bias on the} grid I88 of valve I80 renders the valve more: conducting so that more current flows through, the circuit extending. from the negative terminal of the supply rectifier bridge 200 through the, reactor 2I6, conductor 2I4, potentiometer re sistor I60, conductors l0 and 14, resistor I6, conductor I8, resistors I 02 and 2I2, flexible lead 2T8",v self-biasing resistor 208, cathode [9'0 and anode: I84 of valve I88, and conductor I86 to the pos-- itive terminal of the rectifier bridge 200. The: current flowing through that part of potenti ometer resistor I60 in the grid-cathode circuit. of valves 36 and 38 renders the direct-current biasing potential on the grids I08 and H0, re-] spectively, more negative to block. the firing of the valves 36 and 38 and prevent, the flow of cur rent therethrough to the motor 26.

At the same time that valve I80 becomes conducting, the change in the biasing potential on the grid I88 of valve I82 is such asto' decrease the current passed by the valve I82 with the result that the current flow through the pp" tentiometer resistor I62 is so decreased or be comes zero, that the direct-current biasing potential on the grids H2 and H4 of valves 40 and 42, respectively, becomes less negative or more positive, and the valves 40 and 42' become conducting to pass current to the motor 26. The motor circuit thus established extends from the center tap I06 of the secondary winding of transformer 54 through conductor I84, resistor I822, conductors 18 and 88, armature windings 32 of the motor 26, conductors 82 and 84, resistor I00, conductor 98, the parallel conductors 62 and 9694, cathodes 64 and 42, respectively, and anodes 48 and 58, respectively, of valves 40 and 42, respectively, to the terminals of the secondary winding of the transformer 54 to so energize the, motor 26 as to effect an operation thereof in a direction to raise the electrode I2.

of control resistors 232 and 256 and which is proportional to the are potential, and the direct-,

current voltage across each of resistors 234 and- 258 and which is proportional to the current flow:

ing through the electrode I2 are balanced and:

a zero differential appears across the series con=.

nected resistors 232-234 and the series con nected resistors 256-258. Under such conditions the self-biasing resistors 208 and 2I8 function to maintain suiiicient bias on the grids of valves I80 and I82, respectively, whereby they are sufiiciently conducting to pass suflicient current through potentiometer" resistors I80 and IE2, respectively, that the direct-current biasing potential on the grids of valves 36-38 and 40-42, respectively, is suflicient to block the valves from passing current to the motor 28.

If the ideal operating condition for the arc furnace I is changed in any manner whatsoever, the balance of the direct-current voltages across control resistors 232-234 and 256-258 is upset with the result that the bias on the grids of valves I80 and I82 is changed to efiect an operation of the motor 26 in a predetermined manner to again establish the required balance between the current and potential across the are. If, for example, the electrode I2 should come in contact with the metal I8 as by reason of a cave-in of the metal, the are potential is reduced to a minimum and the current flowing through the electrode is a maximum with the result that the differential of the direct-current voltages across control resistors 232 and 234 is of such a polarity as to render the grid of valve I80 more positive and thereby render the valve I80 more conductive. As described hereinbefore, when the valve I80 is thus rendered more conducting the flow of current through potentiometer resistor I80 so changes the grid-cathode potential of valves 38 and 38 as to render them non-conducting.

At the same time, the differential of the control voltages across resistors 256 and 258 is such as to place a more negative bias on the grid of valve I82 to decrease the current flow through the valve I82. Under such conditions, the directcurrent biasing potential on the valves 40 and 42 is such that the valves are rendered less negative and current flows therethrough to the motor 25 to cause its operation to raise the electrode I2.

During the operation of the system the resistors 16 and I00 function to automatically limit the current flowing to the motor 26, each of the resistors being of low ohmic value and being connected in the motor armature circuits. As illustrated the resistor I00 is also connected in the anode-cathode circuit of valve I82 whereas the resistor I8 is also connected in the anode-cathode circuit of valve I80.

When the motor 26 is at rest the series connected resistors 222, 86 and I00 carry no motor current and therefore they have no effect on the control of valve I82. Likewise the resistors 2 2, I02 and I in the anode-cathode circuit of valve I80 carries no motor current when the motor 26 is at rest. If then, the direct-current voltages across control resistors 232-234 and 256-258 become unbalanced, as for example, whereby the differential of the direct-current voltages is such as to cause a more negative voltage on the grid I88 of valve I82, the negative bias from the associated potentiometer resistor I62 impressed on the grids of valves 40 and 42 is reduced and the motor current flowing therethrough will tend to reach a high value when the motor 26 is started from rest.

Under such conditions, the current flowing through the armature windings 32 also flows through resistor I00 which is also connected in the grid circuit of valve I82. A the flow of cur. rent to the motor 26 through resistor I00 increases, the potential across resistor I00 is such as to apply a positive grid bias to the grid valve I82 to offset s ome of the negative grid bias impressed as a result of the diiferential oi the di rect-current voltages across control resistors 258 and 258 whereby the valve I82 becomes more conducting. As the valve I82 thus becomes more conducting, current flows through the potentiometer resistor I 02 to effect a change in the grid bias of valves 40 and 42 to phase back the firing angle of the valves 40 and 42 to effectively reduce the motor voltage and current. The resistor I00 is so selected that the potential drop thereacross is only effective at motor currents of a predetermined high amplitude or value, for example, such as are encountered at starting from rest or the like, to render the valve I82 more conducting. At all values of motor current below the selected and predetermined value, the resistor I00 is ineffective for rendering the valve I82 more conducting.

The resistor 15 functions in the same manner to limit the motor current if the diiierential of the direct-current voltages across resistors 232 and 234 is such as to cause a more negative voltage on the grid I86 of valve I whereby the valves 36 and 38 are rendered conducting to pass current to the motor 26. Under such conditions the current flowing through armature windings 32 also flows through resistor 16 and where the motor current reaches a predetermined value, a positive grid bias from the resistor I6 is impressed on the grid of valve I80 to offset some of the negative grid bias impressed thereon by the differential of the control voltages to render the valve I80 more conducting. The flow of current through the potentiometer resistor I60 thus resulting ef fects a change in the bias of valves 36 and 38 to phase back the firing angle of the valves 35 and 38 to effectively reduce the motor voltage and current. The resistor I8 is selected in the same manner as resistor I00 so that the potential drop thereacross is only effective at motor currents of a predetermined high value approaching the safe value for the valves 36 and 38 in so offsetting the negative bias of the differential to cause the valve I 80 to conduct. At all values of motor Current below the predetermined value the positive biasing potential of resistor I8 is insufficient to overcome the negative biasing eifect of the differential of the control voltage.

As will be apparent with the system connected as described, if the biasing potential of either of the sets 36-38 or 40-42 of valves becomes less negative, more current flows through the armature windings 32 of the motor 26, the increase in current effecting an increase in the speed of the motor. As the position of the electrode I2 is changed, the diiTerential of control voltages across resistors 232-234 and 258-258 is changed to control the bias on the grids of discharge devices I80 and I82 whereby the bias on valves 36-38 and 40-42 is controlled to control the current passed to the motor 28. Thus as the bias of either set 26-28 or 40-42 becomes less positive the flow of current therethrough decreases to effectively slow the motor as the electrode I2 approaches a, position where a predetermined balance is obtained between the arc current and are potential.

The systemof this invention is effective in affording protection for the valves controlling the fiow of current to the motor and can readily be duplicated. Further, as the system utilizes standard parts it is apparent that the advantages of the system of this invention can readily be obtained without greatly adding to the invest ment of present systems.

I claim as my invention:

1. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the motor being normally disposed to coast when either of the valve means functions to disconnect the motor from the source, a control means for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the control means, and means connected in circuit between each of the valve means and the motor to be responsive to the flow of motor current of a predetermined value to develop a predetermined potential of a predetermined polarity, said means also being connected in circuit between the source of control voltage and the control means and being disposed to effect an operation of the control means when the potential impressed thereon is of the predetermined value to control the valve means conducting current to the motor to maintain the flow of current at a value below a predetermined safe value for the valve means.

2. In a control system, in combination, a motor the speed of which is to be controlled, a pair of electric valves disposed to connect the motor to a source of current and to control the speed of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a control circuit disposed for controlling the bias imposed on the grids, the control circuit including a resistor connected in circuit with the grids for controlling the bias thereof, means including a discharge valve having a control grid therein for supplying a current to the resistor, means comprising a source of control voltage variable in response to changes in the speed of the motor for impressing a variable bias on the control grid of the discharge valve, and a control resistor connected in circuit relation with the motor, source of control voltage and the discharge valve to be responsive to a predetermined flow of current through the motor to control the bias of the discharge valve and the pair of electric valves controlled thereby to maintain the flow of current to the motor at a predetermined safe value for the pair of electric valves.

3. In a control system for regulating the feeding of an electrode for producing and maintaining an electric arc, the combination comprising, a motor the speed of which is to be controlled, a pair of electric valves disposed to connect the motor to a source of current and to control the speed of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for the electric valves, a discharge valve having a control grid therein to be utilized for controlling the flow of current through the resistor to control the bias of the grids of the pair of electric valves, a pair of sources of control voltages variable in opposite senses in response to electrical conditions of the are for normally controlling the bias imposed on the grid of the discharge valve, and a control resistor connected in circuit relation with the motor, the pair of sources of control voltages and the discharge valve, the control resistor thereby being responsive to a predetermined flow of current through the motor to control the bias of the discharge valve and the pair of electric valves controlled thereby to maintain the flow of current to the motor at a predetermined safe value for the pair of electric valves.

4. In a control system for regulating the feeding of an electrode for producing and maintaining an electric arc, the combination comp-rising, a motor disposed for raising and lowering the electrode, a plurality of pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for each pair of the electric valves, a discharge valve having a control grid therein to be utilized for controlling the flow of direct current through the resistor for controlling the bias of the grids of the pair of electric valves associated therewith, a pair of sources of control voltages variable in opposite senses in response to electrical conditions of the arc for impressing predetermined biases on the grids of the discharge valves, and a pair of control resistors, each of the control resistors being connected in circuit relation with the motor, source of control voltage and discharge valve associated with a pair of the electric valves, each of the control resistors being responsive to the flow of motor current of a predetermined value to control the bias of an associated discharge valve and the pair of electric valves controlled thereby to maintain the flow of current to the motor at a safe value for the pair of electric valves.

5. In a control system for regulating the feeding of an electrode for producing and maintain ing an electric arc, the combination comprising, a motor disposed for reversible operation, a plurality of pairs of rectifiers electrically connected to provide a plurality of pairs of sources of control voltages, each pair of sources of control voltages being variable in opposite senses, a pair of discharge valves connected to a source of electrical energy, each of the discharge valves having a grid connected in circuit with a pair of the sources of control voltage, the pair of sources thereby normally controlling the conductivity of the discharge valve connected in circuit therewith. a plurality of pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the speed and direction of operation of the motor, each of the discharge valves being connected in circuit relation to an associated pair of the electric valves to control the conductivity thereof, and a pair of control resistors, each of the control resistors being connected in circuit relation with the motor, a discharge valve and a pair of the sources of control voltage, each of the control resistors being responsive to the flow of motor current of a predetermined value to control the bias of an associated discharge valve and the pair of electric valves controlled thereby to maintain the flow of current to the motor at a safe value for the pair of electric valves.

JACK E. REILLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

